Light emitting diode

ABSTRACT

A light emitting diode includes a first semiconductor layer, an active layer, a second semiconductor layer, an upper electrode, and a lower electrode. The active layer is sandwiched between the first semiconductor layer and the second semiconductor layer. The lower electrode is electrically connected with the first semiconductor layer, and the upper electrode is electrically connected with the second semiconductor layer. A surface of the second semiconductor layer away from the active layer is used as the light extraction surface. A surface of the first semiconductor layer connected with the lower electrode is a patterned surface including a number of grooves.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/288,180, filed on Nov. 3, 2011, entitled, “LIGHT EMITTING DIODE,”which claims all benefits accruing under 35 U.S.C. §119 from ChinaPatent Application 201110110763.8, filed on Apr. 29, 2011 in the ChinaIntellectual Property Office, the disclosure of which is incorporatedherein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a light emitting diode (LED) and amethod for making the same.

2. Description of the Related Art

LEDs are semiconductors that convert electrical energy into light.Compared to conventional light sources, the LEDs have higher energyconversion efficiency, higher radiance (i.e., they emit a largerquantity of light per unit area), longer lifetime, higher responsespeed, and better reliability. At the same time, LEDs generate lessheat. Therefore, LED modules are widely used as light sources in opticalimaging systems, such as displays, projectors, and so on.

A conventional LED commonly includes an N-type semiconductor layer, aP-type semiconductor layer, an active layer, an N-type electrode, and aP-type electrode. The active layer is located between the N-typesemiconductor layer and the P-type semiconductor layer. The P-typeelectrode is located on the P-type semiconductor layer. The N-typeelectrode is located on the N-type semiconductor layer. Typically, theP-type electrode is transparent. In operation, a positive voltage and anegative voltage are applied respectively to the P-type semiconductorlayer and the N-type semiconductor layer. Thus, holes in the P-typesemiconductor layer and electrons in the N-type semiconductor layer canenter the active layer and combine with each other to emit visiblelight.

However, extraction efficiency of LEDs is low because typicalsemiconductor materials have a higher refraction index than that of air.Large-angle light emitted from the active layer may be internallyreflected in LEDs, so that a large portion of the light emitted from theactive layer will remain in the LEDs, thereby degrading the extractionefficiency.

What is needed, therefore, is a light emitting diode and a method formaking the same, which can overcome the above-described shortcomings.

What is needed, therefore, is a light emitting diode that can overcomethe above-described shortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a flowchart of one embodiment a method for manufacturing aLED.

FIG. 2 shows a Scanning Electron Microscope (SEM) image of oneembodiment of a drawn carbon nanotube film.

FIG. 3 shows a schematic view of one embodiment of a carbon nanotubesegment of a drawn carbon nanotube film.

FIG. 4 shows a SEM image of one embodiment of a plurality of carbonnanotube film stacked in a cross order.

FIG. 5 shows a SEM image of one embodiment of an untwisted carbonnanotube wire.

FIG. 6 shows a SEM image of one embodiment of a twisted carbon nanotubewire.

FIG. 7 shows a Transmission Electron Microscopy (TEM) of a cross-sectionof a junction between the first semiconductor layer and the substrate.

FIG. 8 shows a schematic view of one embodiment of a LED made accordingto the method of FIG. 1.

FIG. 9 is a flowchart of another embodiment of a method for making aLED.

FIG. 10 shows a schematic view of one embodiment of a LED made accordingto the method of FIG. 9.

FIG. 11 is a flowchart of another embodiment of a method for making aLED.

FIG. 12 is a flowchart of another embodiment of a method for making aLED.

FIG. 13 shows a schematic view of one embodiment of a LED made accordingto the method of FIG. 12.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

Referring to FIG. 1, a method for manufacturing a light emitting diode(LED) 10 includes the following steps:

-   -   (S11) providing a substrate 100 having an epitaxial growth        surface 101;    -   (S12) placing a first carbon nanotube layer 110 on the epitaxial        growth surface 101;    -   (S13) growing a first semiconductor layer 120, an active layer        130, and a second semiconductor layer 140 in that order on the        epitaxial growth surface 101;    -   (S14) applying an upper electrode 150 on a surface of the second        semiconductor layer 140;    -   (S15) removing the substrate 100; and    -   (S16) applying a lower electrode 160 on a surface of the first        semiconductor layer 120.

In step (S11), the substrate 100 can be made of a transparent materialand adapted to support the first semiconductor layer 120. A shape or asize of the substrate 100 can be selected according to need. Theepitaxial growth surface 101 can be used to grow the first semiconductorlayer 120. The epitaxial growth surface 101 is a clean and smoothsurface. The substrate 100 can be a single-layer structure or amulti-layer structure. If the substrate 100 is a single-layer structure,the substrate 100 can be a single-layer crystal structure having acrystal face used as the epitaxial growth surface 101. If the substrate100 is a multi-layer structure, the substrate 100 includes at least onelayer having the crystal face. The material of the substrate 100 can beGaAs, GaN, AlN, Si, SOI, SiC, MgO, ZnO, LiGaO₂, LiAlO₂, or Al₂O₃. Thematerial of the substrate 100 can be selected according to the materialof the first semiconductor layer 120. The first semiconductor layer 120and the substrate 100 should have a small crystal lattice mismatch and athermal expansion mismatch. The size, thickness, and shape of thesubstrate 100 can be selected according to need. In one embodiment, thesubstrate 100 is a sapphire substrate.

In step (S12), the first carbon nanotube layer 110 includes a pluralityof carbon nanotubes. The carbon nanotubes in the first carbon nanotubelayer 110 can be single-walled, double-walled, or multi-walled carbonnanotubes. The length and diameter of the carbon nanotubes can beselected according to need. The thickness of the first carbon nanotubelayer 110 can be in a range from about 1 nm to about 100 μm, forexample, about 10 nm, 100 nm, 200 nm, 1 μm, 10 μm or 50 μm. The firstcarbon nanotube layer 110 forms a pattern so one part of the epitaxialgrowth surface 101 can be exposed from the patterned first carbonnanotube layer 110 after the first carbon nanotube layer 110 is placedon the epitaxial growth surface 101. Thus, the first semiconductor layer120 can grow from the exposed epitaxial growth surface 101.

The patterned first carbon nanotube layer 110 defines a plurality ofapertures 112. The apertures 112 can be dispersed uniformly. Theapertures 112 extend throughout the first carbon nanotube layer 110along the thickness direction thereof. The aperture 112 can be a holedefined by several adjacent carbon nanotubes, or a gap defined by twosubstantially parallel carbon nanotubes and extending along axialdirection of the carbon nanotubes. The size of the aperture 112 can bethe diameter of the hole or width of the gap, and the average aperturesize can be in a range from about 10 nm to about 500 μm, for example,about 50 nm, 100 nm, 500 nm, 1 μm, 10 μm, 80 μm or 120 μm. Thehole-shaped apertures 112 and the gap-shaped apertures 112 can exist inthe patterned first carbon nanotube layer 110 at the same time. Thesizes of the apertures 112 within the same carbon nanotube layer can bedifferent. The smaller the size of the apertures 112, the lessdislocation defects will occur during the process of growing firstsemiconductor layer 120.

In one embodiment, the sizes of the apertures 112 are in a range fromabout 10 nm to about 10 μm. A duty factor of the first carbon nanotubelayer 110 is an area ratio between the sheltered epitaxial growthsurface 101 and the exposed epitaxial growth surface 101. The dutyfactor of the first carbon nanotube layer 110 can be in a range fromabout 1:100 to about 100:1, for example, about 1:10, 1:2, 1:4, 4:1, 2:1or 10:1. In one embodiment, the duty factor of the first carbon nanotubelayer 110 is in a range from about 1:4 to about 4:1.

The carbon nanotubes of the first carbon nanotube layer 110 can beorderly arranged to form an ordered carbon nanotube structure ordisorderly arranged to form a disordered carbon nanotube structure. Theterm ‘disordered carbon nanotube structure’ includes, but is not limitedto, a structure where the carbon nanotubes are arranged along manydifferent directions, and the aligning directions of the carbonnanotubes are random. The number of the carbon nanotubes arranged alongeach different direction can be substantially the same (e.g. uniformlydisordered). The disordered carbon nanotube structure can be isotropic.The carbon nanotubes in the disordered carbon nanotube structure can beentangled with each other. The term ‘ordered carbon nanotube structure’includes, but is not limited to, a structure where the carbon nanotubesare arranged in a consistently systematic manner, e.g., the carbonnanotubes are arranged approximately along a same direction and/or havetwo or more sections within each of which the carbon nanotubes arearranged approximately along a same direction (different sections canhave different directions).

In one embodiment, the carbon nanotubes in the first carbon nanotubelayer 110 are arranged to extend along the direction substantiallyparallel to the epitaxial growth surface 101 to obtain a better patternand greater light transmission. After being placed on the epitaxialgrowth surface 101, the carbon nanotubes in the first carbon nanotubelayer 110 are arranged to extend along the direction substantiallyparallel to the epitaxial growth surface 101. In one embodiment, all thecarbon nanotubes in the first carbon nanotube layer 110 are arranged toextend along the same direction. In another embodiment, some of thecarbon nanotubes in the first carbon nanotube layer 110 are arranged toextend along a first direction, and some of the carbon nanotubes in thefirst carbon nanotube layer 110 are arranged to extend along a seconddirection, perpendicular to the first direction. Also the carbonnanotubes in the ordered carbon nanotube structure can be arranged toextend along the crystal orientation of the substrate 100 or along adirection which forms an angle with the crystal orientation of thesubstrate 100.

The first carbon nanotube layer 110 can be formed on the epitaxialgrowth surface 101 by chemical vapor deposition (CVD), transfer printinga preformed carbon nanotube film, filtering and depositing a carbonnanotube suspension. In one embodiment, the first carbon nanotube layer110 is a free-standing structure and can be drawn from a carbon nanotubearray. The term “free-standing structure” means that the first carbonnanotube layer 110 can sustain the weight of itself when it is hoistedby a portion thereof without any significant damage to its structuralintegrity. Thus, the first carbon nanotube layer 110 can be suspended bytwo spaced supports. The free-standing first carbon nanotube layer 110can be laid on the epitaxial growth surface 101 directly and easily.

The first carbon nanotube layer 110 can be a substantially purestructure of the carbon nanotubes, with few impurities and chemicalfunctional groups. The first carbon nanotube layer 110 can be acomposite including a carbon nanotube matrix and non-carbon nanotubematerials. The non-carbon nanotube materials can be graphite, graphene,silicon carbide, boron nitride, silicon nitride, silicon dioxide,diamond, amorphous carbon, metal carbides, metal oxides, or metalnitrides. The non-carbon nanotube materials can be coated on the carbonnanotubes of the first carbon nanotube layer 110 or filled in theapertures 112. In one embodiment, the non-carbon nanotube materials arecoated on the carbon nanotubes of the first carbon nanotube layer 110 sothe carbon nanotubes can have a greater diameter and the apertures 112can a have smaller size. The non-carbon nanotube materials can bedeposited on the carbon nanotubes of the first carbon nanotube layer 110by CVD or physical vapor deposition (PVD), such as sputtering.

Furthermore, the first carbon nanotube layer 110 can be treated with anorganic solvent after being placed on the epitaxial growth surface 101so the first carbon nanotube layer 110 can be firmly attached on theepitaxial growth surface 101. Specifically, the organic solvent can beapplied to the entire surface of the first carbon nanotube layer 110 orthe entire first carbon nanotube layer 110 can be immersed in an organicsolvent. The organic solvent can be volatile, such as ethanol, methanol,acetone, dichloroethane, chloroform, or mixtures thereof. In oneembodiment, the organic solvent is ethanol.

The first carbon nanotube layer 110 can include at least one carbonnanotube film, at least one carbon nanotube wire, or a combinationthereof. In one embodiment, the first carbon nanotube layer 110 caninclude a single carbon nanotube film or two or more stacked carbonnanotube films. Thus, the thickness of the first carbon nanotube layer110 can be controlled by the number of the stacked carbon nanotubefilms. The number of the stacked carbon nanotube films can be in a rangefrom about 2 to about 100, for example, about 10, 30, or 50. In oneembodiment, the first carbon nanotube layer 110 can include a layer ofparallel and spaced carbon nanotube wires. The first carbon nanotubelayer 110 can also include a plurality of carbon nanotube wires crossedor weaved together to form a carbon nanotube net. The distance betweentwo adjacent parallel and spaced carbon nanotube wires can be in a rangefrom about 0.1 μm to about 200 μm. In one embodiment, the distancebetween two adjacent parallel and spaced carbon nanotube wires can be ina range from about 10 μm to about 100 μm. The size of the apertures 112can be controlled by controlling the distance between two adjacentparallel and spaced carbon nanotube wires. The length of the gap betweentwo adjacent parallel carbon nanotube wires can be equal to the lengthof the carbon nanotube wire. It is understood that any carbon nanotubestructure described can be used with all embodiments.

In one embodiment, the first carbon nanotube layer 110 includes at leastone drawn carbon nanotube film. A drawn carbon nanotube film can bedrawn from a carbon nanotube array that is able to have a film drawntherefrom. The drawn carbon nanotube film includes a plurality ofsuccessive and oriented carbon nanotubes joined end-to-end by van derWaals attractive force therebetween. The drawn carbon nanotube film is afree-standing film. Referring to FIGS. 2 and 3, each drawn carbonnanotube film includes a plurality of successively oriented carbonnanotube segments 113 joined end-to-end by van der Waals attractiveforce therebetween. Each carbon nanotube segment 113 includes aplurality of carbon nanotubes 115 parallel to each other, and combinedby van der Waals attractive force therebetween. Some variations canoccur in the drawn carbon nanotube film. The carbon nanotubes 115 in thedrawn carbon nanotube film are oriented along a preferred orientation.The drawn carbon nanotube film can be treated with an organic solvent toincrease the mechanical strength and toughness, and reduce thecoefficient of friction of the drawn carbon nanotube film. A thicknessof the drawn carbon nanotube film can range from about 0.5 nm to about100 μm. The drawn carbon nanotube film can be attached to the epitaxialgrowth surface 101 directly.

Referring to FIG. 4, the first carbon nanotube layer 110 can include atleast two stacked drawn carbon nanotube films. In other embodiments, thefirst carbon nanotube layer 110 can include two or more coplanar carbonnanotube films, and each coplanar carbon nanotube film can includemultiple layers. Additionally, if the carbon nanotubes in the carbonnanotube film are aligned along one preferred orientation (e.g., thedrawn carbon nanotube film), an angle can exist between the orientationof carbon nanotubes in adjacent films, whether stacked or adjacent.Adjacent carbon nanotube films are combined by the van der Waalsattractive force therebetween. An angle between the aligned directionsof the carbon nanotubes in two adjacent carbon nanotube films can rangefrom about 0 degrees to about 90 degrees. If the angle between thealigned directions of the carbon nanotubes in adjacent stacked drawncarbon nanotube films is larger than 0 degrees, a plurality ofmicropores is defined by the first carbon nanotube layer 110. Referringto FIG. 4, the first carbon nanotube layer 110 shown with the anglebetween the aligned directions of the carbon nanotubes in adjacentstacked drawn carbon nanotube films is 90 degrees. Stacking the carbonnanotube films will also add to the structural integrity of the firstcarbon nanotube layer 110.

Heating of the drawn carbon nanotube film can be performed to decreasethe thickness of the drawn carbon nanotube film. The drawn carbonnanotube film can be partially heated by a laser or microwaves. Thethickness of the drawn carbon nanotube film can be reduced because someof the carbon nanotubes will be oxidized. In one embodiment, the drawncarbon nanotube film is irradiated by a laser device in an atmosphereincluding oxygen therein. The power density of the laser is greater than0.1×10⁴ W/m². The drawn carbon nanotube film can be heated by fixing thedrawn carbon nanotube film and moving the laser device at aneven/uniform speed to irradiate the drawn carbon nanotube film. When thelaser irradiates the drawn carbon nanotube film, the laser is focused onthe surface of drawn carbon nanotube film to form a laser spot. Thediameter of the laser spot ranges from about 1 micron to about 5 mm. Inone embodiment, the laser device is carbon dioxide laser device. Thepower of the laser device is about 30 W. The wavelength of the laser isabout 10.6 μm. The diameter of the laser spot is about 3 mm. Thevelocity of the laser movement is less than 10 mm/s. The power densityof the laser is about 0.053×10¹² W/m².

In another embodiment, the first carbon nanotube layer 110 can include apressed carbon nanotube film. The pressed carbon nanotube film can be afree-standing carbon nanotube film. The carbon nanotubes in the pressedcarbon nanotube film are arranged along a same direction or arrangedalong different directions. The carbon nanotubes in the pressed carbonnanotube film can rest upon each other. Adjacent carbon nanotubes areattracted to each other and combined by van der Waals attractive force.An angle between a primary alignment direction of the carbon nanotubesand a surface of the pressed carbon nanotube film is about 0 degrees toapproximately 15 degrees. The greater the pressure is applied, thesmaller the angle formed. If the carbon nanotubes in the pressed carbonnanotube film are arranged along different directions, the first carbonnanotube layer 110 can be isotropic.

In another embodiment, the first carbon nanotube layer 110 includes aflocculated carbon nanotube film. The flocculated carbon nanotube filmcan include a plurality of long, curved, disordered carbon nanotubesentangled with each other. Furthermore, the flocculated carbon nanotubefilm can be isotropic. The carbon nanotubes can be substantiallyuniformly dispersed in the carbon nanotube film. Adjacent carbonnanotubes are acted upon by van der Waals attractive force to form anentangled structure with micropores defined therein. It is understoodthat the flocculated carbon nanotube film is very porous. Sizes of themicropores can be less than 10 μm. The porous nature of the flocculatedcarbon nanotube film will increase the specific surface area of thefirst carbon nanotube layer 110. Additionally, because the carbonnanotubes in the first carbon nanotube layer 110 are entangled with eachother, the first carbon nanotube layer 110 employing the flocculatedcarbon nanotube film has excellent durability, and can be fashioned intodesired shapes with a low risk to the integrity of the first carbonnanotube layer 110. In some embodiments, the flocculated carbon nanotubefilm is a free-standing structure because the carbon nanotubes beingentangled and adhered together by van der Waals attractive forcetherebetween.

The carbon nanotube wire can be untwisted or twisted. Treating the drawncarbon nanotube film with a volatile organic solvent can form theuntwisted carbon nanotube wire. Specifically, the organic solvent isapplied to soak the entire surface of the drawn carbon nanotube film.During the soaking, adjacent parallel carbon nanotubes in the drawncarbon nanotube film will bundle together, due to the surface tension ofthe organic solvent as it volatilizes. Thus, the drawn carbon nanotubefilm will be shrunk into untwisted carbon nanotube wire. Referring toFIG. 5, the untwisted carbon nanotube wire includes a plurality ofcarbon nanotubes substantially oriented along a same direction (i.e., adirection along the length of the untwisted carbon nanotube wire). Thecarbon nanotubes are parallel to the axis of the untwisted carbonnanotube wire. Specifically, the untwisted carbon nanotube wire includesa plurality of successive carbon nanotube segments joined end to end byvan der Waals attractive force therebetween. Each carbon nanotubesegment includes a plurality of carbon nanotubes substantially parallelto each other, and combined by van der Waals attractive forcetherebetween. The carbon nanotube segments can vary in width, thickness,uniformity, and shape. Length of the untwisted carbon nanotube wire canbe arbitrarily set as desired. A diameter of the untwisted carbonnanotube wire ranges from about 0.5 nm to about 100 μm.

The twisted carbon nanotube wire can be formed by twisting a drawncarbon nanotube film using a mechanical force to turn the two ends ofthe drawn carbon nanotube film in opposite directions. Referring to FIG.6, the twisted carbon nanotube wire includes a plurality of carbonnanotubes helically oriented around an axial direction of the twistedcarbon nanotube wire. Specifically, the twisted carbon nanotube wireincludes a plurality of successive carbon nanotube segments joined endto end by van der Waals attractive force therebetween. Each carbonnanotube segment includes a plurality of carbon nanotubes parallel toeach other, and combined by van der Waals attractive force therebetween.Length of the carbon nanotube wire can be set as desired. A diameter ofthe twisted carbon nanotube wire can be from about 0.5 nm to about 100μm. Further, the twisted carbon nanotube wire can be treated with avolatile organic solvent after being twisted. After being soaked by theorganic solvent, the adjacent paralleled carbon nanotubes in the twistedcarbon nanotube wire will bundle together, due to the surface tension ofthe organic solvent when the organic solvent volatilizes. The specificsurface area of the twisted carbon nanotube wire will decrease, whilethe density and strength of the twisted carbon nanotube wire will beincreased.

As discussed above, the first carbon nanotube layer 110 can be used as amask for growing the first semiconductor layer 120. The term ‘mask forgrowing the first semiconductor layer 120’ means that the first carbonnanotube layer 110 can be used to shelter part of the epitaxial growthsurface 101 and expose the other part of the epitaxial growth surface101. Thus, the first semiconductor layer 120 can grow from the exposedepitaxial growth surface 101. The first carbon nanotube layer 110 canform a patterned mask on the epitaxial growth surface 101 because thefirst carbon nanotube layer 110 defines a plurality of apertures 112.Compare to lithography or etching, the method of forming a first carbonnanotube layer 110 as a mask is simple, low in cost, and will notpollute the substrate 100.

In step (S13), the first semiconductor layer 120, the active layer 130and the second semiconductor layer 140 can be grown respectively via aprocess of molecular beam epitaxy (MBE), chemical beam epitaxy (CBE),vacuum epitaxy, low temperature epitaxy, choose epitaxy, liquid phasedeposition epitaxy (LPE), metal organic vapor phase epitaxy (MOVPE),ultra-high vacuum chemical vapor deposition (UHVCVD), hydride vaporphase epitaxy (HVPE), or metal organic chemical vapor deposition(MOCVD). In one embodiment, the material of the first semiconductorlayer 120, the active layer 130, and the second semiconductor layer 140are the same semiconductor material, thus the defects caused bydislocation during the growth process will be reduced.

The first semiconductor layer 120 has a thickness of about 0.5 nm toabout 5 μm, for example 10 nm, 100 nm, 1 μm, 2 μm, and 3 μm. In oneembodiment, the thickness of the first semiconductor layer 120 is about2 μm. The first semiconductor layer 120 is N-type semiconductor orP-type semiconductor. The material of N-type semiconductor can includeN-type gallium nitride, N-type gallium arsenide, or N-type copperphosphate. The material of P-type semiconductor can include P-typegallium nitride, P-type gallium arsenide, or P-type copper phosphate.The N-type semiconductor is configured to provide electrons, and theP-type semiconductor is configured to provide holes. In one embodiment,the first semiconductor layer 120 is an N-type gallium nitride dopedwith Si.

In one embodiment, the first semiconductor layer 120 is made by MOCVD,and the growth of the first semiconductor layer 120 is heteroepitaxialgrowth. In the MOCVD, the nitrogen source gas is high-purity ammonia(NH₃), the carrier gas is hydrogen (H₂), the Ga source gas can betrimethyl gallium (TMGa) or triethyl gallium (TEGa), and the Si sourcegas is silane (SiH₄). The growth of the first semiconductor layer 120includes the following steps:

-   -   (S131) placing the substrate 100 with the first carbon nanotube        layer 110 thereon into a reaction chamber, heating the substrate        100 to about 1100° C. to about 1200° C., introducing the carrier        gas, and baking the substrate 100 for about 200 seconds to about        1000 seconds;    -   (S132) growing the low-temperature GaN layer by reducing the        temperature to a range from about 500° C. to 650° C. in the        carrier gas atmosphere, and introducing the Ga source gas and        the nitrogen source gas at the same time;    -   (S133) stopping the flow of the Ga source gas in the carrier gas        and nitrogen source gas atmosphere, increasing the temperature        to a range from about 1100° C. to about 1200° C., and        maintaining the temperature for about 30 seconds to about 300        seconds;    -   (S134) growing the high quality first semiconductor layer 120 by        maintaining the temperature of the substrate 100 in a range from        about 1000° C. to about 1100° C., and reintroducing the Ga        source gas and the Si source gas.

In step (S132), the low-temperature GaN is used as a buffer layer (notshown) to grow the first semiconductor layer 120. The thickness of thebuffer layer is less than the thickness of the first carbon nanotubelayer 110. Because the first semiconductor layer 120 and the substrate100 have different lattice constants, the buffer layer is used to reducethe lattice mismatch during the growth process, thus the dislocationdensity of the first semiconductor layer 120 will be reduced.

In step (S134), the growth of the first semiconductor layer 120 includesthree stages. In the first stage, a plurality of epitaxial crystalnucleus forms on the epitaxial growth surface 101, and the epitaxialcrystal nucleus grows to a plurality of epitaxial crystal grains alongthe direction perpendicular the epitaxial growth surface 101. In thesecond stage, the plurality of epitaxial crystal grains grows to acontinuous epitaxial film along the direction parallel to the epitaxialgrowth surface 101. In the third stage, the epitaxial film continuouslygrows along the direction perpendicular to the epitaxial growth surface101 to form a high quality epitaxial film, the epitaxial growth grains,epitaxial film and the high-quality epitaxial film constitute the firstsemiconductor layer 120.

In the first stage, because the first carbon nanotube layer 110 islocated on the epitaxial growth surface 101, the epitaxial crystalgrains are only grown from the exposed epitaxial growth surface 101through the apertures 112. The process of epitaxial crystal grainsgrowing along the direction substantially perpendicular to the epitaxialgrowth surface 101 is called vertical epitaxial growth.

In the second stage, the epitaxial crystal grains can grow along thedirection parallel to the epitaxial growth surface 101. The epitaxialcrystal grains are gradually joined together to form the epitaxial filmto cover the first carbon nanotube layer 110. During the growth process,the epitaxial crystal grains will grow around the carbon nanotubes, andthen a plurality of grooves 122 will be formed in the firstsemiconductor layer 110 where the carbon nanotubes exist. The extendingdirection of the grooves 122 is parallel to the orientated direction ofthe carbon nanotubes. The carbon nanotubes are located into the grooves122 and enclosed by the first semiconductor layer 120 and the substrate100, thus the carbon nanotubes will be semi-enclosed by the firstsemiconductor layer 120. An inner wall of the grooves 122 can be incontact with the carbon nanotubes or spaced from the carbon nanotubes,which depends on whether the material of the epitaxial film and thecarbon nanotubes have mutual infiltration. Each groove 122 includes atleast one carbon nanotube. The carbon nanotubes in the grooves 122 arejoined by van der Waals force to form the first carbon nanotube layer110. The shape of the grooves 122 correspond to the patterned firstcarbon nanotube layer 110. The maximum width of the grooves 122 rangesfrom about 20 nm to about 200 nm. The maximum width means that themaximum size along the direction perpendicular to the extendingdirection of the grooves 122. In one embodiment, the maximum width ofthe grooves 122 ranges from about 50 nm to about 100 nm. The pluralityof grooves 122 forms a patterned surface on the first semiconductorlayer 120. The patterned surface of the first semiconductor layer 120 issimilar to the first carbon nanotube layer 110.

While the first carbon nanotube layer 110 includes a carbon nanotubefilm or a plurality of intersected carbon nanotube wires, the pluralityof grooves 122 are interconnected with each other to form a continuousnetwork structure. The carbon nanotubes are also interconnected witheach other to form a conductive structure. While the first carbonnanotube layer 110 includes a plurality of carbon nanotube wiresparallel to each other, the plurality of grooves 122 will be parallel toeach other. The grooves 122 are aligned with a certain interval, thedistance between the two adjacent grooves 122 is substantially equal tothe distance between the two adjacent carbon nanotube wires.

Also referring to FIG. 7, a cross-section of a junction between thefirst semiconductor layer 120 and the substrate 100 is shown. Thedark-colored layer is the first semiconductor layer 120, and thelight-colored layer is the substrate 100. The grooves 122 exist on theface of the first semiconductor layer 120. The carbon nanotubes aresemi-enclosed by the grooves 122 and attached on the surface of thesubstrate 100. In one embodiment, the carbon nanotubes are spaced fromthe first semiconductor layer 120.

The active layer 130 is deposited on the first semiconductor layer 120.The thickness of the active layer 130 ranges from about 0.01 μm to about0.06 μm. The active layer 130 is a photon excitation layer and can beone of a single layer quantum well film or multilayer quantum wellfilms. The active layer 130 is made of GaInN, AlGaInN, GaSn, AlGaSn,GalnP, or GalnSn. In one embodiment, the active layer 130 has athickness of about 0.3 μm and includes one layer of GaInN and anotherlayer of GaN. The GaInN layer is stacked with the GaN layer. The growthmethod of the active layer 130 is similar to the first semiconductorlayer 120. In one embodiment, the indium source gas is trimethyl indium.The growth of the active layer 130 after the growth of the firstsemiconductor layer 120 includes the following steps:

-   -   (a1) stopping the flow of the Si source gas and maintaining the        temperature of the reaction chamber in a range from about        700° C. to about 900° C., and the pressure of the reaction        chamber ranges from about 50 torrs to about 500 torrs; and    -   (a2) introducing the indium source gas and growing an InGaN/GaN        multilayer quantum well film to form the active layer 130.

The thickness of the second semiconductor layer 140 ranges from about0.1 μm to about 3 μm. The second semiconductor layer 140 can be anN-type semiconductor layer or a P-type semiconductor layer. Furthermore,the type of the second semiconductor layer 140 is different from thetype of the first semiconductor layer 120. A surface of the secondsemiconductor layer 140 is used as an extraction surface of the LEDs. Inone embodiment, the second semiconductor layer 140 is a P-type galliumnitride doped with Mg. The thickness of the second semiconductor layer140 is about 0.3 μm. The growth of the second semiconductor layer 140 issimilar to the first semiconductor layer 120. The second semiconductor140 is grown after the growth of the active layer 130. In oneembodiment, the Mg source gas is ferrocene magnesium (Cp₂Mg), the methodincludes the following steps:

-   -   (b1) stopping the flow of the indium source gas and maintaining        the temperature of the reaction chamber in a range from about        1000° C. to about 1100° C., and maintaining the pressure of the        reaction chamber to a range from about 76 torrs to about 200        torrs; and    -   (b2) growing P-type gallium nitride doped with Mg to form the        second semiconductor layer 140 by introducing the Mg source gas.

In step (S14), the upper electrode 150 can be an N-type electrode orP-type electrode, the type of the upper electrode 150 is same as thesecond semiconductor layer 140. The shape of the upper electrode 150 isarbitrary and can be selected according to need. The upper electrode 150is located and contacted on a region of the surface of the secondsemiconductor layer 140. The upper electrode 150 is located on theextraction surface of the LED 10. The extraction efficiency of the LED10 is not affected by the shape and the location of the upper electrode150. While the upper electrode 150 is transparent, the upper electrode150 can cover the whole extraction surface. The upper electrode 150 iscan be single layer structure or a multi-layer structure. The materialof the upper electrode 150 can be selected from titanium (Ti), silver(Ag), aluminum (Al), nickel (Ni), gold (Au), or any combination thereof.The material of the upper electrode 150 can also be indium-tin oxide(ITO) or carbon nanotube film. In one embodiment, the upper electrode150 is a P-type electrode and located on one side of the secondsemiconductor layer 140. The upper electrode 150 is a two-layerstructure consisting of a Ti layer with a thickness of about and an Aulayer with a thickness of about 100 nm in thickness.

The upper electrode 150 is placed on the second semiconductor layer 140via a process of physical vapor deposition, such as electron beamevaporation, vacuum evaporation, ion sputtering, or physical deposition.In one embodiment, the upper electrode 150 formed on the secondsemiconductor layer 140 via a physical deposition method includes:

-   -   (S141) coating a layer of photo resist on the top surface of the        second semiconductor layer 140;    -   (S142) removing a portion of the photo resist to expose the        second semiconductor layer 140;    -   (S143) depositing the upper electrode 150 on the top surface of        the second semiconductor layer 140 where the layer of photo        resist has been removed; and    -   (S144) removing the residual photo resist via an organic        solvent, such as acetone to form the upper electrode 150.

In step (S15), the substrate 100 can be removed by laser irradiation,etching, or thermal expansion and contraction. The removal method can beselected according to the material of the substrate 100 and the firstsemiconductor layer 120. In one embodiment, the substrate 100 is removedby laser irradiation. The substrate 100 is removed from the firstsemiconductor layer 120 by the following steps:

-   -   (S151) polishing and cleaning the surface of the substrate 100        away from the first semiconductor layer 120;    -   (S152) placing the substrate 100 on a platform (not shown) and        irradiating the substrate 100 and the first semiconductor layer        120 by a laser; and    -   (S153) immersing the substrate 100 into a solvent to remove the        substrate 100.

In step (S151), the substrate 100 can be polished by a mechanicalpolishing method or a chemical polishing method to obtain a smoothsurface. Thus the scatting of the laser will be reduced. The substrate100 can be cleaned with hydrochloric acid or sulfuric acid to remove themetallic impurities and oil.

In step (S152), the substrate 100 is irradiated by the laser from thepolished surface, and the incidence angle of the laser is perpendicularto the surface of the substrate 100. The wavelength of the laser isselected according to the material of the first semiconductor layer 120and the substrate 100. The energy of the laser is less than the bandgapof the substrate 100 and larger than the bandgap of the firstsemiconductor layer 120. Thus the laser can pass through the substrate100 and reach the interface between the substrate 100 and the firstsemiconductor layer 120. The buffer layer 1202 at the interface has astrong absorption of the laser, and the temperature of the buffer layer1202 will be raised rapidly. Thus the buffer layer 1202 will decompose.In one embodiment, the bandgap of the first semiconductor layer 120 isabout 3.3 eV, and the bandgap of the substrate 100 is about 9.9 eV. Thelaser is a KrF laser, the wavelength of the laser is about 248 nm, andthe energy is about 5 eV, the pulse width range about 20 nanoseconds toabout 40 nanoseconds, the energy density ranges from about 400 mJ/cm² toabout 600 mJ/cm², and the shape of the laser pattern is square with asize of 0.5 mm×0.5 mm. The laser moves from one edge of the substrate100 at a speed of 0.5 mm/s. During the irradiating process, the GaN isdecomposed to Ga and N₂. It is understood that the parameter of thelaser can be adjusted according to need. The wavelength of the laser canbe selected according to the absorption of the buffer layer 1202.

The buffer layer 1202 is decomposed rapidly because the buffer layer1202 has a strong absorption of the laser. However, the firstsemiconductor layer 120 has a weak absorption of the laser, so it cannotbe decomposed. The irradiating process can be performed in a vacuum or aprotective gas environment to prevent the oxidation of the carbonnanotubes. The protective gas can be nitrogen, helium, argon, or otherinert gas.

In step (S153), the substrate 100 can be immersed in an acidic solutionto remove the Ga decomposed from GaN, so that the substrate 100 can bepeeled off from the first semiconductor layer 120. In one embodiment,the first carbon nanotube layer 110 is directly attached on theepitaxial growth surface 101 of the substrate 100, so the carbonnanotubes can be peeled off with the substrate 100 together. During thepeeling process, the shape and the distribution of the grooves are notchanged.

In the thermal expansion and contraction method, the substrate 100 isheated to a temperature above 1000° C. and cooled to a temperature below1000° C. in a short time of about 2 minutes to about 20 minutes. Thesubstrate 100 separates from the first semiconductor layer 110 bycracking because of the thermal expansion mismatch between the substrate100 and the first semiconductor layer 110.

In step (S16), the lower electrode 160 is placed on the firstsemiconductor layer 120 via a process of physical vapor deposition, suchas electron beam evaporation, vacuum evaporation, ion sputtering, orphysical deposition. Furthermore, the lower electrode 160 can be formedby applying a conductive plate on the first semiconductor layer 120 viaa conductive adhesive. The lower electrode 160 can be an N-typeelectrode or P-type electrode. The type of the lower electrode 160 isthe same as the first semiconductor layer 120. The lower electrode 160can be a single layer structure or a multi-layer structure. The materialof the lower electrode 160 can be titanium (Ti), silver (Ag), aluminum(Al), nickel (Ni), gold (Au), or any combination. The lower electrode160 is configured as a reflective layer, a conductive electrode, and aheatsink at the same time. In one embodiment, the lower electrode 160 isa two-layer structure consisting of a Ti layer with 15 nm in thicknessand an Au layer with 200 nm in thickness. The Au layer is attached onthe first semiconductor layer 120. The patterned surface of the firstsemiconductor layer 120 can be partly covered by the lower electrode160. In one embodiment, the whole patterned surface of the firstsemiconductor layer 120 is covered by the lower electrode 160, so thatmore photons can be reflected by the lower electrode 160 and extractedfrom the light extraction surface, thus improving the extractionefficiency of the LED 10. At the same time, the heat produced by the LED10 can be conducted out, thereby decreasing the temperature of the LED10 and prolonging the life of the LED 10.

The method for making the LED 10 has many advantages. First, the carbonnanotube layer is a free-standing structure, thus it can be directlylocated on the surface of the substrate and the complex sputteringprocess is not required. Second, due to the existence of the carbonnanotubes, the plurality of grooves are formed in the LED, thus thecomplex etching method can be avoided and the damage to the latticestructure of the LED is reduced. Third, because the diameter of thecarbon nanotubes and the width of the grooves is so small, theextraction efficiency of the LED is improved. Fourth, the carbonnanotube layer is a graphical structure and the thickness and the sizeof the apertures are small. While it is used to grow the epitaxiallayer, the epitaxial grains will have a smaller size, the dislocationwill be reduced, and the quality of the semiconductor layer will beimproved.

Referring to FIG. 8, an LED 10 includes a first semiconductor layer 120,an active layer 130, a second semiconductor layer 140, an upperelectrode 150, and a lower electrode 160. The active layer 130 issandwiched between the first semiconductor layer 120 and the secondsemiconductor layer 140. The lower electrode 160 is electricallyconnected with the first semiconductor layer 120, and the upperelectrode 150 is electrically connected with the second semiconductorlayer 140. The surface of the second semiconductor layer 140 away fromthe active layer 130 is used as the light extraction surface. Thesurface of the first semiconductor layer 120 which is connected with thelower electrode 160 includes a plurality of grooves 122 to form apatterned surface. The width of the grooves 122 range from about 50 nmto about 100 nm.

When the photons generated from the active layer 130 reaches theplurality of grooves 122 with a large incident angle, the movingdirection of the photons will be changed. After the photons arereflected by the lower electrodes 160, the photons can pass through thelight extraction surface, and the extraction efficiency of the LED 10will be improved.

Referring to FIG. 9, a method for making an LED 20 includes the followsteps:

-   -   (S21) providing a substrate 100, the substrate includes an        epitaxial growth surface 101;    -   (S22) growing a buffer layer 1202 on the epitaxial growth        surface 101;    -   (S23) placing a first carbon nanotube layer 110 on the buffer        layer 1202;    -   (S24) growing a first semiconductor layer 120, an active layer        130, and a second semiconductor layer 140 in that order on the        buffer layer 1202 and the first carbon nanotube layer 110;    -   (S25) depositing a upper electrode 150 on a surface of the        second semiconductor layer 140;    -   (S26) removing the substrate 100 and exposing the first carbon        nanotube layer 110; and    -   (S27) applying a lower electrode 160 on the first semiconductor        layer 120 and electrically connect with the first carbon        nanotube layer 110.

The method for making the LED 20 is similar to the method for making theLED 10 except that the buffer layer 1202 grows on the substrate 100before placing the first carbon nanotube layer 110.

In step (S22), the method of growing the buffer layer 1202 is similar tothe first semiconductor layer 110. The material of the buffer layer 1202is selected from Si, GaAs, GaN, GaSb, InN, InP, InAs, InSb, AlP, AlAs,AlSb, AlN, GaP, SiC, SiGe, GaMnAs, GaAlAs, GaInAs, GaAlN, GaInN, AlInN,GaAsP, InGaN, AlGaInN, AlGaInP, GaP:Zn or GaP:N, according to the firstsemiconductor layer 110. In one embodiment, the buffer layer 1202 is thelow-temperature GaN used to reduce the dislocation of the firstsemiconductor layer 120.

In one embodiment, the buffer layer 1202 is fabricated by a MOCVDmethod. The nitrogen source gas is high-purity NH₃, the carrier gas isH₂, the Ga source gas is TEGa or TEGa. The growth of the buffer layer1202 includes the following steps:

-   -   (S221) placing the substrate 100 into a reaction chamber and        heating the substrate 100 to about 1100° C. to about 1200° C.,        introducing the carrier gas and baking the substrate 100 for        about 200 s to about 1000 s;    -   (S222) cooling down the temperature to a range from about        500° C. to about 650° C. in the carrier gas atmosphere,        introducing the Ga source gas and the nitrogen source gas at the        same time to grow low-temperature GaN layer.

In step (S23), the first carbon nanotube layer 110 is placed on thebuffer layer 1202. The carbon nanotubes are electrically contacted withthe buffer layer 1202. While the first carbon nanotube layer 110 isplaced on the buffer layer 1202, the plurality of carbon nanotubes arealigned parallel to the surface of the buffer layer 1202. The firstcarbon nanotube layer 110 includes a plurality of apertures 112, and thebuffer layer 1202 is exposed from the first carbon nanotube layer 110through the apertures 112.

In step (S24), the Ga source gas is TMGa) or TEGa, the Si source gas isSiH₄, and the method of growing the first semiconductor layer 120includes three stages. In the first stage, a plurality of epitaxialcrystal nucleus forms on the buffer layer 1202, and the epitaxialcrystal nucleus grow a plurality of epitaxial crystal grains along thedirection perpendicular the buffer layer 1202. In the second stage, theplurality of epitaxial crystal grains are joined together to form acontinuous epitaxial film along the direction parallel to the surface ofbuffer layer 1202. In the third stage, the epitaxial film continuouslygrows along the direction perpendicular to the surface of the bufferlayer 1202 to form the first semiconductor layer 120.

In the second stage, during the growth process, the epitaxial crystalgrains will grow around the carbon nanotubes and join together, and aplurality of grooves 122 will be formed in the first carbon nanotubelayer 110 at the carbon nanotubes. The carbon nanotubes are located intothe grooves 122 and enclosed by the first semiconductor layer 120 andthe buffer layer 1202, thus the carbon nanotubes will be semi-enclosedby the first semiconductor layer 120. The surface of the carbonnanotubes will be partly attached on the inner surface of the grooves122. The plurality of grooves 122 form a patterned surface of the firstsemiconductor layer 120. The patterned surface of the firstsemiconductor layer 120 is similar to the first carbon nanotube layer110.

In step (S26), the substrate 100 can be removed by the method mentionedabove. However, the buffer layer 1202 is sandwiched between the firstcarbon nanotube layer 110 and the substrate 100, thus the carbonnanotubes are not directly attached on the surface of the substrate 100and cannot be peeled of with the substrate 100. While the buffer layer1202 is irradiated by the laser, the buffer layer 1202 will bedecomposed, and the buffer layer 1202 will be dissolved in the solution.Thus the first carbon nanotube layer 110 will be detached from thebuffer layer 1202, and the carbon nanotubes will be preserved in thegrooves 122. Due to the buffer layer 1202, damage to the grooves 122will be reduced during the peeling process. The carbon nanotubes canalso decrease the contact surface between the buffer layer 1202 and thefirst semiconductor layer 120, thus the stress will be reduced.

Also referring to FIG. 10, an LED 20 includes a first carbon nanotubelayer 110, a first semiconductor layer 120, an active layer 130, asecond semiconductor layer 140, a lower electrode 150, and an upperelectrode 160. The active layer 130 is sandwiched between the firstsemiconductor layer 120 and the second semiconductor layer 140. Thelower electrode 160 is electrically connected with the firstsemiconductor layer 120, and the upper electrode 150 is electricallyconnected with the second semiconductor layer 140. The surface of thefirst semiconductor layer 120, which connects with the lower electrode160 includes a plurality of grooves 122. The carbon nanotubes of thefirst carbon nanotube layer 110 are embedded into the grooves 122. Thecarbon nanotubes are exposed from the first semiconductor layer 120through the grooves 122 and connected with the lower electrode 160.

Each groove 122 has at least one carbon nanotube therein. The carbonnanotubes in the grooves 122 are joined by van der Waals force to formthe first carbon nanotube layer 110. The surface of the carbon nanotubeswill be partly attached on the inner surface of the grooves 122. Becausethe carbon nanotubes have a strong specific surface, the carbonnanotubes will be fixed in the grooves 122.

In one embodiment, the first carbon nanotube layer 110 is a carbonnanotube film. The carbon nanotube film includes a plurality of carbonnanotubes oriented along a preferred orientation. In the orientation,the carbon nanotubes are joined end to end. In the directionperpendicular to the orientation, a plurality of gaps or micro-holesexist between some adjacent carbon nanotubes. The gaps or micro-holesform the apertures 112. The first carbon nanotube layer 110 includes aplurality of apertures 112. The first semiconductor layer 120 is partlyfilled into the apertures 112.

Furthermore, the first carbon nanotube layer 110 can also include aplurality of carbon nanotube wires parallel with each other. Each of thecarbon nanotube wires is fixed into a groove 122. The distance betweentwo adjacent carbon nanotube wires range from about 0.1 μm to about 200μm. In one embodiment, the distance ranges from about 10 μm to about 100μm. The interval between the adjacent two carbon nanotube wires formsthe apertures 112 of the first carbon nanotube layer 110. The smallerthe size of the apertures 112, the less dislocations will exist in thegrowth of the first semiconductor layer 120, and the quality of thesemiconductor layer 120 will be improved.

In one embodiment, the first carbon nanotube layer 110 can also includea plurality of carbon nanotube wires intersected with each other. Somecarbon nanotube wires extend along a first direction, and some carbonnanotube wires extend along a second direction. The first direction andthe second direction are intersected. In one embodiment, the firstdirection and the second direction are substantially perpendicular witheach other. Thus the surface of the first semiconductor layer 120includes a plurality of grooves 122 intersected with each other.

In the LED 20, the carbon nanotube layer is a free-standing structure,and the carbon nanotubes of the carbon nanotube layer have a largecontact surface with the electrode. Thus, the heat produced by the LEDcan be quickly conducted out of the LED. Furthermore, the conductioncurrent in the LED can be uniformly dispersed.

Referring to FIG. 11, a method for making the LED 20 includes thefollowing steps:

-   -   (S31) providing a substrate 100, wherein the substrate 100        includes an epitaxial growth surface 101;    -   (S32) growing a buffer layer 1202 and an intrinsic semiconductor        layer 1204 in that order on the epitaxial growth surface 101;    -   (S33) placing a first carbon nanotube layer 110 on the intrinsic        semiconductor layer 1204;    -   (S34) growing a first semiconductor layer 120, an active layer        130, and a second semiconductor layer 140 in that order on the        intrinsic semiconductor layer 1204 and the first carbon nanotube        layer 110;    -   (S35) depositing an upper electrode 150 on a surface of the        second semiconductor layer 140;    -   (S36) removing the substrate 100 and exposing the first carbon        nanotube layer 110; and    -   (S37) depositing a lower electrode 160 on a surface of the first        carbon nanotube layer 110.

In step (S32), the method of growing the intrinsic semiconductor layer1204 on the buffer layer 1202 includes the following steps:

-   -   (S321) keeping the temperature of the reaction chamber at a        range from about 1000° C. to about 1100° C. and the pressure in        a range from about 100 torr to about 300 torr;    -   (S322) introducing the Ga source gas and growing the intrinsic        semiconductor layer 1204 on the buffer layer 1202.

In step (S322), the thickness of the intrinsic semiconductor layer 1204ranges from about 10 nm to about 1 μm.

In step (S34), the surface of the intrinsic semiconductor layer 1204 ispartly exposed through the apertures 112 of the first carbon nanotubelayer 110, and the epitaxial grains grow on the surface and pass throughthe apertures 112 to form the first semiconductor layer 120. The activelayer 130 and the second semiconductor layer 140 grow on the surface ofthe intrinsic semiconductor layer 1204 in that order.

In step (S36), during the process of removing the substrate 100 withlaser, the buffer layer 1202 is decomposed and resolved in the acidicsolution, thus the substrate 100 is peeled off. Furthermore, theintrinsic semiconductor layer 1204 can also be decomposed in the acidicsolution at the same time, thus the first carbon nanotube layer 110 willbe exposed. Furthermore, the intrinsic semiconductor layer 1204 can beremoved by ion etching or wet etching.

Because the intrinsic semiconductor layer 1204 grows on the buffer layer1202, thus the dislocations in the first semiconductor layer 120, theactive layer 130 and the second semiconductor layer 140 will be reduced,and the quality of them will be improved. Thus the light extractionefficiency of the LED 20 will be improved.

Referring to FIG. 12, a method for making the LED 30 includes followingsteps:

-   -   (S41) providing a substrate 100, wherein the substrate 100        includes an epitaxial growth surface 101;    -   (S42) growing a buffer layer 1202 on the epitaxial growth        surface 101;    -   (S43) placing a first carbon nanotube layer 110 on the buffer        layer 1202;    -   (S44) growing a first semiconductor layer 120, an active layer        130, a second semiconductor layer 140 in that order on the        buffer layer 1202 and the first carbon nanotube layer 110;    -   (S45) forming a plurality of microstructures 174 on a surface of        the second semiconductor layer 140;    -   (S46) depositing an upper electrode 150 on the surface of the        second semiconductor layer 140 where the microstructures 174 is        located;    -   (S47) removing the substrate 100 and exposing the first carbon        nanotube layer 110; and    -   (S48) depositing a lower electrode 160 on a surface of the first        carbon nanotube layer 110.

The method for making the LED 30 is similar to the method for making theLED 20. The difference is that the method for making the LED 30 furtherincludes a step (S45) of forming a plurality of microstructures 174.

In step (S45), the method of forming the plurality of microstructurescan be lithography or growth method. Referring to FIG. 12, in oneembodiment, the microstructures 174 are formed by the following steps:

-   -   (S451) placing a second carbon nanotube layer 180 on the surface        of the second semiconductor layer 140;    -   (S452) growing a third semiconductor layer 170 on the second        conductor layer 140 and the second carbon nanotube layer 180;        and    -   (S453) removing the second carbon nanotube layer 180 located on        the second semiconductor layer 140.

In step (S451), the second carbon nanotube layer 180 is configured asthe mask layer to grow the third semiconductor layer 170. The structureof the second carbon nanotube layer is same as that of the first carbonnanotube layer 110. The third semiconductor layer 170 can only grow fromthe apertures 112 of the second carbon nanotube layer 180. The secondcarbon nanotube layer 180 includes a plurality of apertures 112, thusthe second carbon nanotube layer 180 can be configured as the patternedmask layer. While the second carbon nanotube layer 180 is located on thesecond semiconductor layer 140, the carbon nanotubes of the secondcarbon nanotube layer 180 is parallel to the surface of the secondsemiconductor layer 140.

In step (S452), a plurality of epitaxial grains grows on a surface ofthe second semiconductor layer 140. The growth direction of theepitaxial grains is perpendicular to the surface of the secondsemiconductor layer 140. During the growth process, the microstructures174 are formed on the surface of the second semiconductor layer 140. Thesurface of the semiconductor layer 140 on which the microstructures 174is located is used as the light extraction surface of the LED. The shapeof the microstructures 174 is the same as the shape of the apertures112. In one embodiment, the carbon nanotubes of the second carbonnanotube layer 180 are oriented substantially along the same direction,thus the shape of the microstructures 174 is in a shape of bar. Themicrostructures 174 are parallel with each other and spaced from eachother. The microstructures 174 are oriented substantially along the samedirection and parallel to the surface of the second semiconductor layer140. The oriented direction of the microstructures 174 is the same asthe carbon nanotubes thereof. The microstructures 174 constitute thethird semiconductor layer 170. The thickness of the third semiconductorlayer 170 is about 2 μm. A slot 172 is formed between the adjacent twomicrostructures 174. The maximum width of the slot 172 ranges from about20 nm to about 200 nm. The carbon nanotubes of the second carbonnanotube layer 180 are located in the slot 172.

In one embodiment, the second carbon nanotube layer 180 includes aplurality of carbon nanotube films intersected with each other or aplurality of carbon nanotube wires intersected with each other. Theepitaxial grains grow from the apertures 112 to form a plurality ofdot-like microstructures 174. The dot-like microstructures 174 aredispersed on the surface of the second semiconductor layer 140. Themaximum size of the dot-like microstructures 174 ranges from about 10 nmto about 10 μm.

The material of the microstructures 174 is arbitrary, and can be GaN,GaS and Cu₃P₂. The material of the microstructures 174 can be same asthe second semiconductor layer 140. In one embodiment, the material ofthe microstructures 174 is GaN doped with Mg.

In step (S453), the second carbon nanotube layer 180 can be removed byplasma etching, ultrasonic oscillation, laser heating, or reactionchamber heating. In one embodiment, the second carbon nanotube layer 180is removed by laser heating. The method of removing the second carbonnanotube layer 180 includes the following steps:

-   -   (c1) providing a laser device, irradiating the second carbon        nanotube layer 180 with the laser transmitted by the laser        device;    -   (c2) scanning the second carbon nanotube layer 180 with the        laser in an oxidized atmosphere.

The laser device can be solid lasers, liquid lasers, gas lasers, orsemiconductor lasers. The power density of the laser is greater than0.053×10¹² watt/m². The diameter of the light spot ranges from about 1mm to about 5 mm. The irradiation time is less than 1.8 second. In oneembodiment, the laser device is CO₂ laser, the power density is about 30watt, the wavelength is about 10.6 μm and the diameter of the light spotis about 3 mm.

The carbon nanotubes on the second semiconductor layer 140 can beablated by the laser. The irradiation time of the laser can becontrolled by controlling the moving speed of the laser relative to thesecond carbon nanotube layer 180. The carbon nanotubes will be oxidizedto CO₂. The greater the laser power and the slower the moving speed, thegreater the energy absorbed by the carbon nanotubes and the quicker theablation. In one embodiment, the moving speed of the laser is about 10mm relative to the carbon nanotube layer. The second carbon nanotubelayer 180 can be scanned by the laser in a direction parallel to theoriented direction of the carbon nanotubes. The scanning direction canalso be perpendicular to the oriented direction of the carbon nanotubes.

The upper electrode 150 is placed on the second semiconductor layer 140via a process of physical vapor deposition, such as electron beamevaporation, vacuum evaporation, ion sputtering, or physical deposition.In one embodiment, the upper electrode 150 is formed on the secondsemiconductor layer 140 via electron beam evaporation method. The secondsemiconductor layer 140 includes a plurality of microstructures 174. Aslot 172 is defined between every two adjacent microstructures 174.During the process of placing the upper electrode 150 on the secondsemiconductor layer 140, one part of the upper electrode 150 isdeposited on the microstructures 174, and another part is deposited inthe caves 172 and connected with the second semiconductor layer 140.Thus the upper electrode 150 is electrically connected with the secondsemiconductor layer 140.

Furthermore, the second carbon nanotube layer 180 is not removed fromthe second semiconductor layer 140, and the second carbon nanotube layer180 will remain in the slots 172. Thus the upper electrode 150 can beformed on the third semiconductor layer 170 and electrically connectedwith the second carbon nanotube layer 180. The second carbon nanotubelayer 180 is conductive, so it can be functional as an electrode todisperse the current flowing in the LED 30.

The method of forming the microstructures on the light extractionsurface of the LED via the carbon nanotube layers has many advantages.One is the method is simple and the cost is lower compared with theetching and nano-imprint lithography method. Another is that the carbonnanotube layer is a free-standing structure and can be directly placedon the third semiconductor layer, thus it can be conveniently used inlarge-scale industrial production.

Referring to FIG. 13, a LED 30 includes a first carbon nanotube layer110, a first semiconductor layer 120, an active layer 130, a secondsemiconductor layer 140, a lower electrode 150, and an upper electrode160. The active layer 130 is sandwiched between the first semiconductorlayer 120 and the second semiconductor layer 140. The lower electrode160 is electrically connected with the first semiconductor layer 120,and the upper electrode 150 is electrically connected with the secondsemiconductor layer 140. The surface of the first semiconductor layer120 is connected with the lower electrode 160 and includes a pluralityof grooves 122. The carbon nanotubes of the second carbon nanotube layer180 are embedded into the grooves 122. The carbon nanotubes are exposedfrom the first semiconductor layer 120 through the grooves 122 andconnected with the lower electrode 160. A plurality of microstructures174 is formed on the surface of second semiconductor layer 140, awayfrom the active layer 130.

The plurality of microstructures 174 are located on the light extractionsurface of the LED 30 and spaced from each other. The shape of themicrostructures 174 can be a bar or a dot. The extending direction ofthe bar-shaped microstructures 174 can be substantially parallel orintersect with the grooves 122. An angle between the microstructures 174and the grooves 122 ranges from about 0 degrees to about 90 degrees. Inone embodiment, the angle is about 90 degrees, thus the extendingdirection of microstructure 174 is substantially perpendicular to thegrooves 122. The width of the microstructure 174 ranges from about 10 nmto about 10 μm. A slot 172 is formed between the two adjacentmicrostructures 174. The maximum width of the slot 172 ranges from about20 nm and 200 nm. In one embodiment, the maximum width of the slots 172ranges from about 50 nm to about 100 nm.

The LED 30 includes a plurality of microstructures located on the lightextraction surface. As the photons arrive at the light extractionsurface with a large angle, the emergence angle of the photons will bechanged due to diffraction, and the photons can extract from the LED 30,thus the light extraction efficiency will be improved. Furthermore, asecond carbon nanotube layer 180 (not shown) is embedded in the slots172, thus the emergence angle of the photons can also be changed by thecarbon nanotube layer 180.

The method for making the LED has many advantages. First, the carbonnanotube layer is a continuous and free-standing structure, and it canbe directly placed on the substrate to grow the epitaxial layer, so thecomplex sputtering process is avoided. Second, a plurality ofmicrostructures can be formed on the light extraction surface of LED viataking carbon nanotube layers as the mask layer, so the complex etchingprocess can be avoided. Third, because the apertures in the carbonnanotube layer and the microstructures is very small, the lightextraction efficiency will be improved. Lastly, because the etchingprocess is avoided, damage to the lattice structure of the LED will bereduced.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and that order of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Variations may be madeto the embodiments without departing from the spirit of the disclosureas claimed. It is understood that any element of any one embodiment isconsidered to be disclosed to be incorporated with any other embodiment.The above-described embodiments illustrate the scope of the disclosurebut do not restrict the scope of the disclosure.

What is claimed is:
 1. A light emitting diode, comprising: a firstsemiconductor layer having a patterned surface defining a plurality ofgrooves, wherein a width of each of the plurality of grooves ranges fromabout 50 nanometers to about 100 nanometers; a second semiconductorlayer having a light extraction surface; an active layer sandwichedbetween the first semiconductor layer and the second semiconductorlayer, wherein the light extraction surface of the second semiconductorlayer and the patterned surface of the first semiconductor layer areaway from the active layer; an upper electrode electrically connectedwith the second semiconductor layer; and a lower electrode electricallyconnected with the patterned surface, wherein the lower electrode coversthe entire patterned surface of the first semiconductor layer.
 2. Thelight emitting diode of claim 1, wherein the plurality of grooves areinterconnected with each other to form a continuous network structure.3. The light emitting diode of claim 1, wherein the plurality of groovesare parallel to each other.
 4. The light emitting diode of claim 1,wherein a part of the lower electrode on each of the plurality ofgrooves is suspended.
 5. The light emitting diode of claim 4, whereinthe part of the lower electrode is spaced from a bottom surface of eachof the plurality of grooves.
 6. The light emitting diode of claim 4,wherein the plurality of grooves are covered by the lower electrode toform an enclosed space.
 7. The light emitting diode of claim 1, whereinthe lower electrode functions as a reflective layer to reflect thephotons.
 8. The light emitting diode of claim 1, further comprising aplurality of carbon nanotubes embedded in the plurality of grooves. 9.The light emitting diode of claim 8, wherein the plurality of carbonnanotubes are connected with each other to form a first carbon nanotubelayer, and the first carbon nanotube layer is a free-standing structure.10. The light emitting diode of claim 9, wherein the plurality of carbonnanotubes extend substantially parallel to the surface of the firstcarbon nanotube layer.
 11. The light emitting diode of claim 9, whereinthe plurality of carbon nanotubes are parallel with the lower electrodeand orderly aligned.
 12. The light of emitting diode of claim 11,wherein the plurality of carbon nanotubes are joined end-to-end by vander Waals attractive force.
 13. The light emitting diode of claim 9,wherein the first carbon nanotube layer defines a plurality of aperturesand a part of the first semiconductor layer contacts the lower electrodethrough the apertures.
 14. A light emitting diode, comprising: a firstsemiconductor layer defining a plurality of grooves, wherein a width ofeach of the plurality of grooves ranges from 50 nanometers to 100nanometers; a second semiconductor layer having a light extractionsurface and comprising a plurality of microstructures formed on thelight extraction surface; an active layer sandwiched between the firstsemiconductor layer and the second semiconductor layer; and a lowerelectrode electrically connected with the first semiconductor layer andcovering the plurality of grooves.
 15. The light emitting diode of claim14, wherein each microstructure is a bar-shape microstructure ordot-shape microstructure.
 16. The light emitting diode of claim 15,wherein an extending direction of the bar-shape microstructures issubstantially parallel to the grooves.
 17. The light emitting diode ofclaim 15, wherein the extending direction of the bar-shapemicrostructures intersects the grooves.